1. Field of the Invention
This invention relates to a process and apparatus for improving the formation of the stock in a paper machine.
In the operation of the typical Fourdrinier paper making machine, a thin suspension of stock fibers in water is flowed from a head box slice onto the upper surface of a moving endless woven screen belt (Fourdrinier wire or fabric) made of metal or plastic material. The fabric passes over a breast roll at one end of the forming section of the machine, and a couch roll at the other. As the fabric travels in contact with spaced-apart transverse dewatering supports (table rolls, foils or blades) and over suction boxes located between the breast and couch rolls, water is withdrawn from the stock through the fabric leaving a thin self-supporting formation of matted fibers on the upper surface. This sheet of formed fibers is lifted off the fabric at the couch roll, at the downstream end of the forming section, and the belt, after travelling around the couch roll, is returned through a series of return rolls to the upstream end of the forming section where it travels around the breast roll and again passes under the slice to complete the cycle.
The paper mill stock supplied to the forming fabric of the machine is made up of fibers and solids in an aqueous suspension containing generally from about 99% to 99.5% water. Despite attempts to thoroughly mix the stock in the head box of the paper machine so that the fibers will be uniformly dispensed, the fibers invariably tend to agglomerate as they emerge from the slice and are deposited on the fabric in clumps or flocs. If these flocs of fibers remain undispersed the finished paper will not be of uniform density. Normally, the fibers tend to remain oriented horizontally. Also, as the stock layer advances through its dewatering path, while the lower strata are drawn ahead by frictional forces acting between them and the forming fabric, the upper strata are less influenced and, through inertia, tend to cause the fibers to form laminae in the stock.
2. Description of the Prior Art
Several methods have been tried to redistribute fibers in the stock after it has been transferred to the forming fabric and during the early stages of dewatering. Some such methods employ mechanical means for shaking components of the machine either laterally or vertically. Others employ air or water jets playing on the stock layer. Usually all these methods have some disadvantage or other.
While it is desirable, in some cases, to provide some means of shaking the stock horizontally in the cross machine or transverse direction, it is also important to provide some rapid vertical displacement to make use of surface instabilities in the wet stock and the resulting shear forces set up within the stock suspension to cause redistribution of fibers. Normally vertical displacement is caused, in the conventional drainage table of a paper machine, by the creation of vacuum as the fabric leaves the supporting surfaces of table rolls or foils. This vacuum forces the Fourdrinier fabric to deflect vertically at the downstream sides of these components thereby causing vertical undulation of the fabric as it passes from one roll or foil to another roll or foil.
In the case of table rolls, the force causing deflection of the fabric increases in intensity with increasing machine speed, and eventually, at high speeds, acts to the detriment of paper quality. In the case of foils, the same condition persists but is controllable to a greater degree by simply decreasing the drainage angle of the foil or altering blade width and spacing. However, on low speed machines it is often difficult to obtain adequate agitation by using foils.